Polyethylenes are important items of commerce, these being produced in larger volumes than any other polymer. Many different grades of this polymer type are produced, these differing grades varying in many properties, including cost. For an overview of polyethylenes, see B. Elvers, et al., Ed., Ullmann's Encyclopedia of Industrial Chemistry, 5th Ed., Vol. A21, VCH Verlagsgesellschaft, Weinheim, 1992, p. 488-518; and H. Mark et al., Ed., Encyclopedia of Polymer Science and Engineering, Vol. 6, John Wiley & Sons, New York, 1986, p. 383-489.
Aside from cost the two major property areas of concern to most users (polymer processors) are final polymer physical properties, that is does the polymer have physical properties suitable for the end use, and how difficult is it to form the polymer into the final article, often called processability. In some instances polyethylene compositions having desirable properties are obtained by blending two or more polyolefins, at least one of which is a polyethylene, together. These blends may be formed by melt blending the separate blend polymers, or by forming two or more of the blend polymers in a single polymerization system (these may be sequential or simultaneous polymerizations). The latter is often preferred because such blends are often more uniform, and the cost of a separate mixing step is avoided.
Blends may be formed to improve physical properties and/or processing properties. For instance, high density polyethylene is sometimes not as tough as desired, so it may be blended with a less crystalline polymer, such as a lower melting (even elastomeric) copolymer of ethylene and an α-olefin (or other polyolefin), to toughen the resulting product. Therefore improved methods of making blends of polyethylene polymers are of interest.
Various reports of “simultaneous” oligomerization and polymerization of ethylene to form (in most cases) branched polyethylenes have appeared in the literature, see for instance WO90/15085, WO99/50318, U.S. Pat. Nos. 5,753,785, 5,856,610, 5,686,542, 5,137,994 and 5,071,927; C. Denger, et al, Makromol. Chem. Rapid Commun., vol. 12, p. 697-701 (1991), and E. A. Benham, et al., Polymer Engineering and Science, vol. 28, p. 1469-1472 (1988). All of the above are incorporated by reference herein for all purposes as if fully set forth.
None of these references specifically describes any of the processes or branched homopolyethylenes of the present invention.